Saturday, 25 August 2007

LHC: The First Year

The New Physics workshop is at its peak. The TH seminar room is cracking in its seams and there are at least two talks every day. Unfortunately, the theory talks last week were ranging from not-so-exctiting to pathetic. Therefore I clench my teeth and report on a talk by an experimentalist. Fabiola Gianotti was talking about the ATLAS status and plans for physics with first data.

Experimentalists (much as happy families) are all alike.
They can never resist showing us a hundred spectacular pictures of their cherished detectors and another hundred showing the tunnel at sunrise and sunset. They feel obliged to inform how many kilometers of cable was wasted for each particular part of a detector. They stuff each cm^2 of the transparancies with equally indispensable pieces of information. Usually, this leaves little space for interesting physics. This time, however, was slightly different. Having finished with the pictures, Fabiola told us several interesting things about early physics with the ATLAS detector.

ATLAS is already alive, kicking and collecting data from cosmic-ray muons. The LHC will go online in Spring 2008. That is to say, if all goes smoothly, if nothing else explodes and if Holger Nielsen refrains from pulling the cards. In the unlikely case of everything going as planned, the first collisions at 14 TeV will take place in July. The plan for ATLAS is to collect a hundred inverse picobarn of data by the end of the year, and a few inverse femtobarn by the end of 2009.

The main task in 2008 will be to discover the Standard Model. 100/pb translates roughly to 10^6 W bosons and to 10^5 Z boson. The decays of W and Z have been precisely measured before, so this sample can be used for callibrating the detectors. We will also see some 10^4 top-antitop pairs with fully- or semi-leptonic decays. Thus, the top will be detected on european soil and we will know for sure that the Tevatron didnt make it up. In the first year, however, the top samples will serve no purpose other than callibration. For example, the top mass resolution will be of order 10 GeV (the ultimate precision is 1 GeV but this is a song of the future), far worse than the current Tevatron sensitivity. Apart from that, the QCD jets background at high pT will be measured, something notoriously difficult to estimate theoretically. In short, 2008 studies will be boring but necessary to prepare the stage for future discoveries.

Is there any hope for a discovery in 2008? Fabiola pointed out the case of a 1 TeV vector resonance decaying to an electron pair. This would stand out like a lamppost over the small and well-understood Drell-Yann background and could be discovered with as little as 70/pb of data. The problem is that LEP and LEP2 put an indirect constraint on the mass of such a resonance to be larger than a few TeV. So not much hope for that.

Another hope for an early discovery is supersymmetry with its multitude of coloured particles. From the plot we can see that a gluino lighter than 1.5 TeV could be discovered with 100/pb of data. A quick discovery would be important, as it would set the green light for the ILC project. The problem in this case is that a discovery requires a good understanding of the missing energy spectra. Most likely, we will have to wait till 2009.

The higgs boson is a more difficult case. From the plot below you can see that there is no way to see anything at 100/pb. However, with a few inverse picobarns the whole interesting range of higgs masses will be covered. Thus, according to Fabiola, the higgs puzzle should be resolved by the end of 2009.

Last thing worth mentioning is the ongoing effort to visualize the huge kinetic energy that will be stored in the LHC beam. This time the energy was compared to that of a British aircraft carrier running at 12 knots. The bottom line is the following. If you spot an aircraft carrier on Lac Leman, don't panic, it's just the LHC that lost the beam.

The slides are available via the workshop page here.


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L. Riofrio said...

Neat and informative post. However, the photo is of an American Nimitz-class aircraft carrier. These ships displace 5 times as much mass as a typical British Invincible-class carrier, therefore have 5 times the kinetic energy at 12 knots.

Jester said...

Thanks Louise. I extracted the picture from Fabiola's transparancies. I hope that the LHC people are not always that careless, otherwise everything will go up in flames ;-)